Scalable silicon anodes and the role of parylene films in improving electrode performance characteristics in energy storage systems

ABSTRACT

A lithium-based energy storage system includes an electrolyte and an electrode. The electrode has a conformal coating of parylene. The parylene forms an artificial solid electrolyte interface (SEI). The electrode may include a material chosen from silicon, graphene-silicon composite, carbon-sulfur, and lithium. The use of parylene to form a conformal coating on an electrode in a lithium-based energy storage system is also disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 15/326,736 filed Jan. 17, 2017, which is a national phase entryof International Patent Application No. PCT/US2015/042635 filed Jul. 29,2015, which claims the benefit of U.S. Provisional Patent ApplicationNo. 62/031,169 filed Jul. 31, 2014, the contents of which areincorporated by reference as if disclosed herein in their entireties.

FIELD OF THE INVENTION

The present invention relates to electrodes for use in energy storagesystems.

BACKGROUND OF THE INVENTION

Research and development activities in advanced energy storage systemscapable of high energy densities have assumed primary importance overthe last few years owing to a drastic rise in feature-intensive consumerelectronics, the advent of electric vehicles as well as the need tosecure a reliable grid storage system for the society. A number ofelectrode materials and battery chemistries have already beeninvestigated by the community in an effort to identify the best solutionfor such high energy demands. Some of the most promising technologies inthis regard include (a) silicon anodes in lithium ion batteries, (b)lithium sulfur batteries, and (c) lithium air batteries. However, thepath to commercialization of these materials and technologies is stillconfronted with significant fundamental challenges.

Lithium Ion Batteries

One of the most common energy storage systems in use today is thelithium ion battery. Graphitic anodes are typically incorporated incommercial lithium ion batteries. Graphite is capable of a nettheoretical capacity of 370 mAh/g, translating to an energy density of100-200 Wh/kg. In addition to a relatively low net theoretical capacity,graphitic anodes also suffer from high irreversible first cycle capacityloss. During the first lithium insertion and extraction cycle (i.e.,lithiation/delithiation), the electrolyte reacts to form anelectrochemical interface with the anode to form what is known as asolid electrolyte interface or SEI. The irreversible loss of electrolyteand lithium in the first cycle leads to a loss of 20% or more of thetheoretical capacity of the energy storage system. The SEI, however,forms a protective barrier to prevent further reaction between theelectrolyte and anode and subsequent cycle losses are low.

Silicon anodes have been studies as an alternative to graphite. Siliconpossesses a net capacity as high as 4200 mAh/g with the potential tooffer energy densities that are an order of magnitude higher thancommercial graphitic anodes, rendering silicon an ideal alternative tographite. However, silicon anodes present different challenges that haveyet to be overcome. During lithium insertion and extraction, siliconundergoes tremendous volume expansion/contraction on the order of280-400% depending on the structure of the silicon. This expansion andcontraction causes the structures to fail prematurely throughpulverization and delamination.

Additionally, silicon forms an unstable SEI with the electrolyte whichleads to extensive loss of active materials. Unlike graphitic anodes, inwhich a stable SEI is formed during the first cycle and prevents furtherreaction between the electrolyte and anode in subsequent cycles, the SEIon silicon anodes breaks down and reforms during eachlithiation/delithiation cycle, leading to substantial loss of capacity.

Attempts at countering the effects of the expansion/contraction effectsof silicon have been directed to nanostructuring the silicon. Nanoscalesilicon has been used to limited success. Current nanostructured siliconanodes use nanowires, which have a tendency to fan out or fold back onthemselves, which leads to a decrease in the space between thenanowires. Therefore, the current nanostructured silicon anodes arelimited to 300 nm thickness before the expansion and contraction of thesilicon leads to pulverization of the silicon.

Delamination of the silicon is also an issue, causing the silicon todelaminate from the underlying current collector, rendering the siliconuseless.

The resulting effect of the aforementioned limitations is thatsilicon-based anodes generally suffer from poor cycle life and drasticcapacity fade, making them unstable for commercial applications.

Lithium Sulfur Batteries

Lithium sulfur batteries offer a theoretical capacity as high as 1700mAh/g and a theoretical energy density as high as 2600 Wh/kg and havebeen considered to be an ideal solution for grid storage.Commercialization of lithium sulfur batteries is however significantlyconstrained by the precise chemical reactions that occur between lithiumand sulfur at the carbon-sulfur cathode site.

Lithium sulfur batteries store energy through the interaction of lithiumand sulfur that eventually form lithium sulfides (Li₂S). However, priorto the formation of lithium sulfides, the chemical reaction initiallyproduces lithium polysulfides (Li₂S₈, Li₂S₆, Li₂S₄, and Li₂S₂). Lithiumpolysulfides are generally soluble in the electrolyte and tend to flowout of the cathode and dissolve in the electrolyte. This process iscommonly referred to as lithium polysulfide dissolution and causessignificant loss of active material, poor recharging capacity, andlimited cycle life, thereby limiting its adoption by the industry. Asshown in FIG. 4, ˜42% of the capacity is contributed by the formation ofvarious lithium polysulfides while the remaining capacity is contributedby insoluble lithium sulfides. The large percentage of capacitycontributed by the lithium polysulfides accounts for a large drop incapacity due to lithium polysulfide dissolution. FIG. 5 shows thevoltage profile of a standard lithium sulfur battery.

In addition, lithium sulfur batteries also undergo expansion andcontraction during the charge/discharge cycle. When a lithium sulfurbattery is completely discharged, the volume of sulfur expands as muchas 200%.

Researchers have attempted to address the issues of lithium sulfurbatteries by altering the battery chemistry to avoid formation oflithium polysulfides in the first place or by confining lithiumpolysulfides within nanoscopic pores in the carbon-sulfur cathode toprevent dissolution in the electrolyte. These methods, however, can becomplex and are economically or environmentally feasible.

Lithium Air Batteries

Lithium air batteries have been considered to be an ideal alternative tolithium ion batteries in automotive applications owing to theirexcellent theoretical energy densities (11,140 Wh/kg), which approachesthe practical achievable energy densities of an internal combustionengine.

Lithium air batteries also have the ability to implement unlimitedambient air as the active reaction species (thereby offering a potentialto lower the cost/kWh significantly), and the fast reaction kinetics ofthe lithium-oxygen interaction allows high power densities to beachieved when required.

The mechanism in a lithium air battery involves the flow of lithium ionsfrom a lithium anode to a carbon-based air cathode where it reacts withoxygen in ambient air to produce lithium peroxide and lithium oxide, asshown in the equations below,

4Li⁺O₂+4e

2Li₂O  (1); and

2Li⁺O₂+2e

Li₂O₂  (2)

The reactions occur at the air cathode site. However, using ambient airalso exposes lithium metal to undesired side reactions with moisture andcarbon dioxide that causes it to irreversibly form lithium hydroxide andlithium carbonate at the anode, thereby limiting its cycle life and netachievable capacities. This further adds to concern regarding safetycharacteristics of the lithium anode in lithium air batteries.

The use of ambient air also poses a fire hazard in lithium airbatteries. Lithium is highly combustible and reacts with water to formhydrogen. Therefore, if a lithium air battery is punctured or damaged,there is a risk that the electrolyte will leak and the lithium anode isexposed to ambient air and the moisture present in the air causing thelithium to combust.

The present invention attempts to solve one or more of the problems withcurrent energy storage systems and provide energy storage systems thathave improved capacity and fade resistance.

SUMMARY OF THE INVENTION

One aspect of the present invention relates to an electrode for anenergy storage system comprising a material chosen from silicon,graphene-silicon composite, carbon-sulfur, and lithium, wherein thematerial has a coating of parylene.

Another aspect of the present invention relates to an energy storagesystem comprising an electrolyte and an electrode, wherein the electrodecomprises a parlyene coating.

Yet another aspect of the present invention relates to an energy storagesystem comprising a nanostructured silicon electrode. The nanostructuredsilicon may have a thickness of at least 300 nm and/or a void density ofat least 20%.

Still another aspect of the present invention relates to a method ofmaking an electrode for an energy storage system comprising providing amaterial chosen from silicon, graphene-silicon composite, carbon-sulfur,and lithium, and forming a coating of parylene on the material.

A further aspect of the present invention relates to the use of paryleneas a coating of an electrode in an energy storage system to reduceinitial capacity fade.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows (a) cross section and (b) top view SEM images of ananostructured silicon having a spiral geometry in accordance with anembodiment of the invention.

FIG. 2 shows (a) a cross section SEN image of nanostructured siliconspirals deposited via 45° step rotation on top of a thin film ofchromium in accordance with an embodiment of the invention; (b) acomparison of discharge capacities of nanostructured silicon spiralswith and without a chromium adhesion promoting layer.

FIG. 3 shows (a) nanostructured silicon spirals coated with parylenewith and without annealing in accordance with an embodiment of theinvention; (b) discharge characteristics of nanostructured siliconspirals coated with Parylene N and Parylene C coatings and uncoatednanostructured silicon spirals.

FIG. 4 shows the loss in achievable capacity in lithium sulfur batteriesusing graphene-wrapped sulfur cathodes.

FIG. 5 shows the voltage profile of a graphene-wrapped sulfur tested asa cathode in a lithium sulfur battery.

FIG. 6 shows the voltage profile of a graphene-silicon composite anodeagainst a lithium cobalt oxide cathode in accordance with an embodimentof the invention.

FIG. 7 shows the first cycle capacity loss of silicon-carbon compositeanodes that have been annealed and pre-lithiated in accordance with anembodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

As used herein, the term “parylene” refers to any poly(xylylene)polymer. Examples of parylenes includes, but not limited to, parylene N,parylene C, and parylene AF-4, which have the following structures:

Electrodes according to embodiments of the present invention comprise acoating of parylene. Parylene can form a conformal coating on theelectrode regardless of the shape of the electrode. The parylene coatingmay be formed by any known method, such as, for example, chemical vapordeposition.

Parylene does not react with electrolytes typically used in energystorage systems, such as lithium-based storage systems including lithiumion batteries, lithium sulfur batteries, and lithium air batteries.Additionally, lithium ions can easily diffuse through the parylenecoating.

Because parylene can form a coating free of pinholes, the parylenecoating may form a physical barrier preventing contact between theelectrode and the electrolyte. Without wishing to be bound by theory, itis believed that the parylene coating forms an artificial solidelectrolyte interface (SEI). Therefore, the parylene coating may reducethe loss in capacity resulting from the formation of an SEI layer bypreventing the electrolyte from reacting with the electrode. Inconventional electrodes, SEI formation can lead to first cycle losses of20% or more.

The parylene coating may also provide structural rigidity to theelectrode. For example, parylene C has a resistance to tensileelongation of as much as 300%. In lithium ion batteries, silicon expandsand contracts during the lithiation/delithiation cycle by ˜280-400%,which leads to delamination and pulverization of the silicon. By coatinga silicon electrode with a conformal coating of parylene, the parylenecan provide structural rigidity to the silicon electrode and preventdelamination and pulverization.

In lithium sulfur batteries, lithium polysulfides (Li₂S₈, Li₂S₆, Li₂S₄,and Li₂S₂) are initially produced before lithium sulfide (Li₂S) isformed. The lithium polysulfides are soluble in the electrolyte and flowout of the carbon-sulfur cathode. This lithium polysulfide dissolutioncauses significant loss of active material, poor recharging capacity,and limited cycle life.

A parylene coating on the carbon-sulfur cathode in a lithium sulfurbattery may prevent the flow of lithium polysulfides out of thecarbon-sulfur cathode. By containing the lithium polysulfides, theactive material is contained within cathode, which may allow the lithiumpolysulfides to form lithium sulfide and maintain the rechargingcapacity and cycle life of the battery.

Parylene is also a hydrophobic material. In lithium air batteries, wherecontact between the lithium and moisture from the air can lead tocombustion, the parylene coating may form a waterproof barrier.

According to at least one embodiment, the parylene coating has athickness ranging from about 1 nm to about 20 nm, such as, for example,from about 5 nm to about 20 nm, or from about 10 nm to about 20 nm. Thethickness of the parylene coating can depend on the desired properties.A thicker coating may provide additional protection against contactbetween the electrolyte or other compounds and the electrode, and athinner coating may minimize the amount of material used and minimizethe diffusion rate of the lithium ions through the parylene coating. Athinner parylene coating may also have less effect on the gravimetricenergy density of the electrode. In some embodiments, a coating lessthan 1 nm may be used, and in other embodiments, a coating greater than20 nm may be used.

The parylene may be coated in the desired thickness, or the parylene maybe annealed after deposition to reduce the thickness through loss ofcarbon during annealing.

Another aspect of the present invention relates to nanostructuredsilicon electrodes.

According to at least one embodiment, the electrode may comprisenanostructured silicon. Nanostructuring the silicon may allow forexpansion and contraction of the silicon during thelithiation/delithiation cycles. The present inventors have found thatnanostructuring by itself does not necessarily provide resistance topulverization through expansion and contraction. Structures such asnanowires can fan out or fold back on themselves, reducing the spaceavailable for expansion and contraction.

In at least one embodiment, the nanostructured silicon has a voiddensity of at least 15%, such as, for example, at least 20%, at least25%, or at least 30%. In other embodiments, the void density may begreater. A greater void density provides more room for expansion of thenanostructured silicon during lithiation.

As used herein, the terms “void density” and “porosity” are usedinterchangeably to describe the amount of space within thenanostructured silicon. For example, nano-rods having a diameter of 50nm spaced 25 nm apart would have a void density of greater than 33%.

In at least one embodiment, the nanostructured silicon has a thicknessof greater than 300 nm. In at least one further embodiment, thenanostructured silicon has a thickness of at least 1 μm or more.

According to at least one embodiment, the nanostructured silicon has anelectrode mass loading of at least 0.5 mg/cm², such as, for example, 1mg/cm² or at least 2 mg/cm². In at least one embodiment, thenanostructured silicon has an electrode mass loading of 2 to 5 mg/cm².

The geometry of the nanostructured silicon is not limited. Thenanostructured silicon can have the shape of rods, wires, springs,spirals, pillars, spheres, etc. In at least one embodiment, thenanostructured silicon has a spiral structure. The spiral structure mayprovide the nanostructured silicon with the ability to longitudinallyexpand during lithiation and delithiation process.

Nanostructured silicon may be formed by any known method. For example,nanostructured silicon can be formed using physical vapor deposition(PVD) techniques such as sputtering and e-beam deposition.

In another embodiment, the nanostructured silicon may comprise siliconparticles. The particles may be bound to a surface, such as a currentcollector or an adhesion promoting surface using a binder. Afterdeposition on the surface, the nanoparticles may then be coated with aparylene coating.

According to at least one embodiment, the electrode comprises anadhesion promoting layer. The adhesion promoting layer may improve theadhesion of the electrode material and the current collector. Forexample, an adhesion promoting layer comprised of chromium or titaniummay be used to improve the adhesion of silicon to a current collectormade of copper. The adhesion promoting layer may be selected based onthe adhesion properties of the current collector and electrode material.Chromium is an inactive material in lithium ion batteries and does notparticipate in lithium intercalation or alloying kinetics and is hencefree from volume changes during charge/discharge. Chromium also displaysexcellent charge transfer characteristics that may improve the ratecapability.

The adhesion promoting layer may be applied as a thin film. For example,the adhesion promoting layer may having a thickness ranging from 1 nm toabout 50 nm, such as about 5 nm to about 30 nm. In other embodiments,the adhesion promoting layer may be thinner than 1 nm or thicker than 50nm depending on the materials used.

In at least one embodiment, the electrode may comprise a carbon-siliconcomposite, such as a graphene-silicon composite. Other forms of carbonmay also be used, including, but not limited to, nanotubes, fullerenes,and pyrolytic graphite. In the carbon-silicon composite, the carbon maycoat the silicon.

According to at least one embodiment, a graphene-silicon composite maybe formed by preparing a solution of graphene oxide dispersed in ethanolor water at concentration ranging from 1 mg/mL to 20 mg/ml, and addingthe dispersion to silicon nanoparticles. In at least one embodiment, thesilicon nanoparticles may have a particle size ranging from 2 nm to 4μm. The ratio of graphene oxide to silicon may be varied between 5%:95%to 95%:5% (by weight). Graphene oxide, with its oxygen moieties, tendsto wrap around the silicon nanoparticles, interacting with the nativeoxide layer of the silicon nanoparticles, and forms a coating. Theviscous suspension of graphene oxide-silicon composite can then beapplied to a metallic current collector (copper, aluminum, nickel,etc.). The suspension can be applied using any of the knownmanufacturing techniques including but not limited to (a)doctor-blading, (b) slot-die coating, (c) spray deposition, and (d)electrophoretic deposition. The graphene oxide-silicon composite maythen be reduced by application of thermal or photo-thermal energy, asdescribed in U.S. Patent Application Publication No. 2014/0050910, whichis hereby incorporated by reference.

Alternatively, the graphene oxide-silicon composite may be reduced priorto its application current collector. The ethanol suspension can bedried out to obtain graphene oxide-silicon composite in powder form, andthen reduced using thermal or photo-thermal energy on the powder. It isalso understood that reduction of graphene oxide can be performed in aliquid phase as well, using various chemical techniques. The reductionprovides a graphene-silicon composite material that may be used as ananode in a lithium ion battery configuration. In addition, a conformalthin layer of parylene may be coated on to the graphene-silicon orgraphene oxide-silicon composite.

The graphene-silicon composite can be annealed to help control capacityloss. For example, the carbon-silicon compositions may be annealed at atemperature ranging from 300° C. to 900° C. under a flowing inert gas,such as, for example, argon, nitrogen, or helium. The carbon-siliconcomposition may be annealed for about 1 to 6 hours.

In accordance with at least one embodiment, following the annealingtreatment, the anodes may be pre-lithiated by bringing them in contactwith a lithium metal foil, in the presence of an electrolyte and underthe application of a compressive force. The annealed and pre-lithiatedanodes can then be assembled in a half-cell (against a lithium metalfoil) or full-cell (against commercial cathodes) configuration.

The annealing and/or pre-lithiation treatment may help prevent thecapacity loss. In at least one embodiment, annealing and/orpre-lithiation treatment may also be used with other anode materialsincluding carbon, tin, tin oxide, aluminum, germanium, silicon, andcomposites of the same.

EXAMPLES

Nanostructured Silicon

Micron long silicon spirals were grown through conventional physicalvapor deposition techniques (specifically, sputtering and e-beam) asshown in FIG. 1. The spirals displayed an intrinsic spring constant thatallowed for its volume change in the longitudinal direction. The siliconspirals did not display the fanning out phenomenon observed in nanowiresand hence significantly longer structures could be fabricated whileeffectively maintaining the space between adjacent structures.

The spiral geometry alone allowed for longer cycling as compared tofilms and nano rods of similar thickness when the thickness wasmaintained below 300 nm. Beyond this 300 nm thickness, delamination dueto poor adhesion at the silicon-current collector interface began toplay a dominant role, leading to a rapid loss in capacity.

Adhesion Promoting Layer

In order to improve adhesion of silicon, a thin film (˜30 nm) ofchromium was deposited onto a copper current collector prior todeposition of the silicon spirals. The silicon spirals were thendeposited on top of the chromium layer. Chromium was found to enhancethe adhesion between silicon and the current collector, improving thecycling ability considerably. Incorporation of a very thin layer ofchromium does not add significantly to the mass of the anode and thus,the gravimetric energy density and power density were not affected.

Adding a chromium adhesion promoting layer enabled 70% retention incapacity at the end of 50 cycles of charge/discharge (see FIG. 2). Incomparison, silicon spirals without a chromium layer displayed almost 0%capacity retention at the end of 50 cycles of charge/discharge.

Parylene Coating on Nanostructured Silicon Spirals

Parylene-N was initially tested as a coating layer for silicon spirals.Different thicknesses of parylene and annealing conditions were testedwith the objective being to identify the thinnest optimum coating thatwould suppress SEI formation while simultaneously allowing lithium ionsto diffuse through and accommodating volume expansion of silicon.

Incorporation of a parylene coating in the previous example furtherimproved the capacity retention to 80% after 100 charge/discharge cyclesat a rate of 0.5 C (see FIG. 3). These results were attributed topassivating characteristics of parylene that would effectively inhibitthe formation of an SEI layer and in effect, act as an artificial SEIlayer that had been pre-formed with the anode. This was confirmed byfundamental electrochemical impedance spectroscopy (EIS) studies thatrevealed an interfacial resistance of 169Ω that remained stablethroughout cycling. Moreover, the charge transfer resistance of parylenecoated silicon was as low as 21Ω, thereby suggesting that in addition toSEI inhibition, parylene also accommodated the volume change in siliconand allowed for efficient intercalation between silicon and lithium.

In addition to Parylene-N, Parylene-C was also tested for itseffectiveness in inducing a stable electrochemical interface andstructural stability to silicon. Parylene-C has a resistance to tensileelongation of as much as 300% and is also a passivating agent and wouldthus continue to inhibit the formation of an SEI layer.

Silicon-Carbon Composite

Graphene-silicon composites synthesized according to the methoddisclosed above provided energy densities in excess of at least 400Wh/kg and power densities of at least 200 W/kg (in a half-cellconfiguration against a lithium metal foil) and a volumetric energydensity of at least 500 Wh/L (in a full-cell configuration against alithium cobalt oxide cathode). In a full-cell configuration against astandard lithium cobalt oxide or lithium iron phosphate cathode, thegraphene-silicon composite anodes worked efficiently within the regularoperating window of lithium ion batteries (3-4.2 V) (see FIG. 6). Asshown in FIG. 7, carbon-silicon composite anodes that were annealed andpre-lithiated in accordance with methods disclosed herein displayed afirst-cycle capacity loss as low as 15%.

While preferred embodiments of the invention have been shown anddescribed herein, it will be understood that such embodiments areprovided by way of example only. Numerous variations, changes, andsubstitutions will occur to those skilled in the art without departingfrom the spirit of the invention. Accordingly, it is intended that theappended claims cover all such variations as fall within the spirit andscope of the invention.

What is claimed is:
 1. A method of making an electrode for an energystorage system, comprising: providing a material chosen from silicon,graphene-silicon composite, carbon-sulfur, or lithium; and forming acoating of parylene on the material.
 2. The method according to claim 1,wherein providing a material chosen from silicon, graphene-siliconcomposite, carbon-sulfur, or lithium comprises coating silicon withgraphene oxide and reducing the graphene oxide to form agraphene-silicon composite.
 3. The method according to claim 2, whereinthe graphene-silicon composite has a thickness greater than 300 nm. 4.The method according to claim 1, wherein providing a material chosenfrom silicon, graphene-silicon composite, carbon-sulfur, or lithiumcomprises forming nanostructured silicon having a void density of atleast 20%.
 5. The method according to claim 4, wherein the void densityis at least 30%.
 6. The method according to claim 4, wherein thenanostructured silicon has a thickness greater than 300 nm.
 7. Themethod according to claim 4, wherein the nanostructured silicon has aspiral geometry.
 8. The method according to claim 1, wherein theparylene is chosen from parylene N, parylene C, or parylene AF-4.
 9. Themethod according to claim 1, wherein the parylene coating has athickness ranging from about 1 nm to about 20 nm.
 10. The methodaccording to claim 1, further comprising: providing a current collector;and providing an adhesion promoting layer positioned between the currentcollector and the material chosen from silicon, graphene-siliconcomposite, carbon-sulfur, or lithium.
 11. The method according to claim10, wherein the adhesion promoting layer comprises chromium or titanium.